The present invention relates to reflective colour filters, and more particularly to cholesteric filters for use in connection with displays devices. The invention also relates to a display device utilising such filters.
Cholesteric materials, also known as chiral nematic materials, can be used as reflective colour filters. One advantage of such filters is that they do not absorb any light, in contrast to conventional absorbing colour filters. Thus, a higher filtering efficiency may be achieved when using cholesteric filters rather than conventional, absorbing colour filters. Furthermore, manufacturing of cholesteric colour filters is less complicated than manufacturing of absorbing colour filters, in the respect that fewer processing steps are typically needed. However, the colour reflected from cholesteric colour filters is strongly dependent on the viewing angle (i.e. the angle of incidence on the filter). The angular acceptance bandwidth of display devices of the reflective type based on cholesteric liquid crystals is typically around xc2x120xc2x0, at most.
A layer of cholesteric material ordered in a planar state (principal axis perpendicular to the surface of the filter layer) acts as an interference film that reflects light of a wavelength matching the pitch of the cholesteric structure. Light not meeting the interference criterion is transmitted through the material. Thus, in order to reflect one specific wavelength, the cholesteric material must have a pitch matching that specific wavelength.
When the angle of incidence is greater than zero, i.e. at an oblique angle of incidence, the effective pitch of the cholesteric material is changed. By consequence, the colour reflected is altered accordingly by the angle of incidence, as mentioned above. The wavelength xcexmax for which maximum reflection occurs is given by:
xcexmax=navgp cos(xcex1),
where navg is the average refractive index of the material, p is the pitch of the cholesteric structure and xcex1 is the angle of incidence with respect to the layer normal. It should be noted that, in the above equation, refraction at the layer surface between the cholesteric material and the ambient is neglected.
Clearly, as the angle of incidence increases, the wavelength of the light reflected decreases. In other words, when a cholesteric filter is viewed under increasing angles, the wavelength of reflected light decreases. For display devices, this angle dependency is very disturbing. For the red component of a multi-colour display, this is particularly annoying, since the colour turns yellow or green. In the context, it should be noted that the human eye is most sensitive for green light. Therefore, the change from red towards green becomes more apparent when a colour display based on cholesteric filters is viewed under an oblique angle. Also, the reproduction of red colour is deteriorated, leading to a loss of colour space.
One previously proposed solution to overcome the above-described problem is to introduce an absorbing filter, which absorbs the unwanted colours. However, such an approach to the problem has several limitations. Firstly, it reintroduces the absorbing colour filters. As mentioned above, one advantage of using reflective filters (such as cholesteric filters) is the possibility of avoiding the expensive and complicated absorbing filters. Secondly, an absorbing filter will influence other colours as well, thereby skewing the colour reproduction. A cholesteric filter layer comprising absorbing dyes which absorb unwanted colours is disclosed in WO 00/33129.
Hence, in the art of reflective, cholesteric colour filters, there is a need for improvements regarding the colour reproduction at oblique viewing angles.
It is a general object of the present invention to provide a solution to the above-described problems regarding the colour shift of cholesteric filters when viewed under oblique angles. This object is achieved by a device and a method of the kind presented in the appended claims.
Hence, the present invention provides a reflective filter structure arranged to reproduce colour by reflection, said structure comprising a first domain adapted to reflect, at normal angle of incidence, light having a red colour, said filter structure being characterised by further comprising a second domain adapted to reflect, at normal angle of incidence, light having an infrared colour, wherein said second domain is further adapted to reflect, at a predetermined angle of incidence greater than zero, light having a red colour.
In particular, it is an object of the present invention to increase the acceptable viewing angle of colour display devices based on reflective filters comprising a material having a cholesteric order.
Thus, it is an object of the present invention to provide a reflective colour filter based on cholesterically ordered liquid crystals, which filter exhibits an enhanced colour reproduction at oblique viewing angles without the need to incorporate absorbing elements. Nevertheless, it can be advantageous to combine the features of the present invention with absorbing elements or dyes in order to achieve particular, desired effects.
The present invention is based on the recognition that the addition of an infrared domain to a conventional RGB pattern of a matrix display can be used for the reproduction of red colour when the display is viewed under oblique angles. The infrared domain is adapted to reflect, when viewed under a normal angle of incidence, light within the infrared range of the colour spectrum. Thus, under a normal (i.e. perpendicular) viewing angle, the infrared domain is invisible to the human eye. However, when viewed under an oblique angle, the wavelength of light reflected from the infrared domain is shifted towards shorter wavelengths. Consequently, the colour reproduction from the infrared domain can be arranged to fall within the red range of the colour spectrum when said domain is viewed under an oblique angle. By virtue of the infrared domain becoming visible red when viewed under oblique angles, the defective yellow or green colour is compensated.
Although the invention is described by means of examples employing cholesteric liquid crystals, it is to be understood that other types of reflective colour filters are possible within the scope of the invention.
One advantage of the reflective filter structure according to the present invention is that the additional infrared domain can be manufactured with existing technology. The colour reflected by a cholesteric film is determined by the pitch of said film. Thus, an infrared domain could easily be incorporated in a matrix display by providing a domain of cholesteric material having a pitch that is different from the pitch of the red, green and blue domains, for example. The present invention gives important enhancement to reflective colour filters without introducing new and complicated processing steps.
Advantageously, the infrared domain is formed in a layer that is common to all domains, i.e. the domains are formed side by side in a single layer. Alternatively, the infrared domain is formed in a second layer on top of the layer comprising the red, green and blue domains. In the latter case, the infrared domain is formed solely on top of the red domain, leaving the green and blue domains uncovered. Similarly, the infrared domain can also be formed in a second layer underneath the red domain.
It should be noted that the terms xe2x80x9cinfraredxe2x80x9d, xe2x80x9credxe2x80x9d, xe2x80x9cgreenxe2x80x9d and xe2x80x9cbluexe2x80x9d domains refer to the colours reflected at a normal (i.e. perpendicular) viewing angle. However, the fact being appreciated that the reflected colour shifts to shorter wavelengths as the viewing angle increases.
In one aspect, the present invention provides a reflecting filter structure of cholesteric material, wherein the reproduction of red colour is augmented for oblique viewing angles. To this end, a reflective filter structure comprises both a red domain and an infrared domain. At zero angle of incidence (i.e. under normal viewing angle), the red domain reflects light of red colour, and the infrared domain reflects light of infrared colour. At an oblique angle of incidence, the infrared domain reflects light of red colour, while the red domain reflects light of yellowish-green colour. As an example only, the infrared domain can be adapted to reflect light within the red range of the colour spectrum when the filter is viewed at an angle of 45xc2x0. Thus, the reproduction of red colour is enhanced at an oblique viewing angle by the presence of the infrared domain.
In another aspect, the present invention provides a pixel structure for full colour reproduction, the pixel structure comprising a blue domain, a green domain, a red domain and an infrared domain. Preferably, the area of each domain is carefully selected in order to reproduce the best possible colour space. Different selections of domain sizes and domain layout will be further described in the following detailed description of some preferred embodiments of the present invention.
Manufacturing of patterned layers of cholesteric material is known in the art. For example, WO 00/34808 discloses a method of manufacturing a patterned layer of a polymer material having a cholesteric order, in which the material is oriented is such a way that the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer.